In silicon based devices such as silicon photovoltaic cells, the metal-silicon interface at the metal contacts provides a highly recombination active surface for minority charge carriers. This may result in recombination currents and degradation of the device performance. For example, the open-circuit voltage Voc of photovoltaic cells decreases with increasing recombination at the metal contacts. Reduction of recombination currents at metal contacts is considered to be a critical aspect of enhancing efficiencies of highly efficient silicon photovoltaic cells.
The saturation current density of metallized semiconductor (e.g., silicon) junctions is a measure for the magnitude of the recombination current and thus for the effective surface recombination velocity of the metallized junction.
Recombination at a metal-emitter interface of a silicon photovoltaic cell, for example, may be determined by extracting the saturation current density of the metallized emitter from Jsc-Voc (short circuit current density—open circuit voltage) measurements and from dark current measurements as a function of metal coverage. Such measurements may need to be done at a device level, which means that it may only be possible to do the measurements on finished devices. This is generally a disadvantage since devices may be complicated and may require many process steps to finish. Using this method, it may not be possible to separate the influence of individual process steps on the recombination at the metal contacts.
It may be desirable to be able to determine surface recombination characteristics at metallized surfaces without the need for full device processing. This would allow gathering recombination information at different stages of the processing and separating the influence of individual process steps on the recombination characteristics. Exact knowledge of the injection level is important for such measurements, since various contributions to the recombination current are injection level dependent.
Photoluminescence measurements can be done at different stages of the processing and have the advantage of spatial resolution, but obtaining information on the injection level may only be possible by using complicated algorithms. This can compromise the reliability of saturation currents extracted using such techniques.
Photo-conductance decay measurements allow for effective lifetime extraction at different injection levels. Such measurements have been used for the determination of the (emitter) saturation current density at high injection levels. When using this approach for the measurement of the saturation current density at a metal/silicon interface, care should be taken to prevent wafer conductivity to be dominated by the metal layer. Therefore, a very thin metal layer (e.g., an Al layer with a thickness of about 1 nm) can be used to permit a photo-conductance decay measurement on such structures. It is a disadvantage that such thin aluminum layers may be fully oxidized before measurements are done, since aluminum is known to react with oxygen in ambient air to form aluminum oxide. Furthermore, there are many metallization techniques that do not allow for the deposition of such thin metal layers.
Photo-conductance calibrated photoluminescence imaging has been used for the local determination of saturation current densities of highly doped regions in silicon wafers, as reported by J. Mueller et al. in “Reverse saturation current density imaging of highly doped regions in silicon: A photoluminescence approach,” Solar Energy Materials & Solar Cells 106 (2012) 76-79. The saturation current density is determined under high injection conditions based on photo-conductance calibrated photoluminescence images acquired at different high injection levels. By providing an optical short pass filter in front of the camera detecting the luminescence photons, the technique can also be applied to partially metallized samples. Using this measurement method the recombination at metal contacts to highly doped silicon can be quantified. It is a disadvantage of this method that saturation currents can only be extracted at high injection levels, and that the determination of injection levels requires calibration with contactless photo-conductance decay measurements and the use of both long and short wavelength pass filters in front of the detector. The thinner the wafer, the shorter the wavelength at which the cut-off needs to be made. Therefore, the method may not be suitable for measurements on thin wafers.